better test for pandas-dev#26796

Author: cchwala

pandas/core/generic.py

@@ -6894,7 +6894,9 @@ def replace(
             Update the data in place if possible.
         limit_direction : {'forward', 'backward', 'both'}, default 'forward'
             If limit is specified, consecutive NaNs will be filled in this
-            direction.
+            direction. If the methods 'pad' or 'ffill' are used it must be
+            None or 'forward'. If 'backfill' or 'bfill' are use it must be
+            None or 'backwards'. 
         limit_area : {`None`, 'inside', 'outside'}, default None
             If limit is specified, consecutive NaNs will be filled with this
             restriction.
@@ -7082,7 +7084,7 @@ def interpolate(
         axis=0,
         limit=None,
         inplace=False,
-        limit_direction="forward",
+        limit_direction=None,
         limit_area=None,
         max_gap=None,
         downcast=None,
@@ -7123,6 +7125,26 @@ def interpolate(
                 "column to a numeric dtype."
             )
 
+        # Set `limit_direction` depending on `method`
+        if (method == 'pad') or (method == 'ffill'):
+            if (limit_direction == 'backward') or (limit_direction == 'both'):
+                raise ValueError(
+                    "`limit_direction` must not be `%s` for method `%s`" % (limit_direction, method)
+                )
+            else:
+                limit_direction = 'forward'
+        elif (method == 'backfill') or (method == 'bfill'):
+            if (limit_direction == 'forward') or (limit_direction == 'both'):
+                raise ValueError(
+                    "`limit_direction` must not be `%s` for method `%s`" % (limit_direction, method)
+                )
+            else:
+                limit_direction = 'backward'
+        else:
+            # Set default
+            if limit_direction is None:
+                limit_direction = 'forward'
+
         # create/use the index
         if method == "linear":
             # prior default

pandas/tests/series/test_missing.py

@@ -1397,6 +1397,17 @@ def test_interp_limit_area(self):
         with pytest.raises(ValueError, match=msg):
             s.interpolate(method="linear", limit_area="abc")
 
+    def test_interp_limit_area_with_pad(self):
+        # Test for issue #26796 -- using `limit_area` with `method=pad`
+        s = Series([np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, np.nan, np.nan])
+        expected = Series([np.nan, np.nan, 3, 3, 3, 3, 7, np.nan, np.nan])
+        result = s.interpolate(method="pad", limit_area="inside")
+        tm.assert_series_equal(result, expected)
+
+        expected = Series([np.nan, np.nan, 3, np.nan, np.nan, np.nan, 7, 7, 7])
+        result = s.interpolate(method="pad", limit_area="outside")
+        tm.assert_series_equal(result, expected)
+
     def test_interp_limit_direction(self):
         # These tests are for issue #9218 -- fill NaNs in both directions.
         s = Series([1, 3, np.nan, np.nan, np.nan, 11])
@@ -1422,6 +1433,38 @@ def test_interp_limit_direction(self):
         result = s.interpolate(method="linear", limit=1, limit_direction="both")
         tm.assert_series_equal(result, expected)
 
+    def test_interp_limit_direction_with_pad_error(self):
+        # Since `pad` forces a forward fill and `bfill` forces a backward fill
+        # they should not be used together with `limit_direction`
+        s = Series([1, 3, np.nan, np.nan, np.nan, 11])
+
+        with pytest.raises(
+                ValueError, match="`limit_direction` must not be `backward` for method `pad`"
+        ):
+            s.interpolate(method="pad", limit=1, limit_direction="backward")
+
+        with pytest.raises(
+                ValueError, match="`limit_direction` must not be `backward` for method `ffill`"
+        ):
+            s.interpolate(method="ffill", limit=1, limit_direction="backward")
+
+        with pytest.raises(
+                ValueError, match="`limit_direction` must not be `both` for method `ffill`"
+        ):
+            s.interpolate(method="ffill", limit=1, limit_direction="both")
+
+        with pytest.raises(
+                ValueError,
+                match="`limit_direction` must not be `forward` for method `backfill`"
+        ):
+            s.interpolate(method="backfill", limit=1, limit_direction="forward")
+
+        with pytest.raises(
+                ValueError,
+                match="`limit_direction` must not be `forward` for method `bfill`"
+        ):
+            s.interpolate(method="bfill", limit=1, limit_direction="forward")
+
     def test_interp_limit_to_ends(self):
         # These test are for issue #10420 -- flow back to beginning.
         s = Series([np.nan, np.nan, 5, 7, 9, np.nan])